1. Field of Invention
The present invention relates to a reflective mask blank for EUV (Extreme Ultraviolet) lithography (hereinafter referred to also as “EUV mask blank” in this specification) to be used for e.g. production of semiconductors, and a process for its production. Further, the present invention relates to a reflective layer-coated substrate to be used for EUV lithography, particularly a reflective layer-coated substrate to be used for the production of the EUV mask blank, and a process for its production.
2. Discussion of Background
Heretofore, in the semiconductor industry, a photolithography method employing visible light or ultraviolet light has been used as a technique to transfer a fine pattern required to form an integrated circuit with a fine pattern on e.g. a silicon substrate. However, the conventional photolithography method has come close to its limit, while miniaturization of semiconductor devices is being accelerated. In the case of the photolithography method, the resolution limit of a pattern is about ½ of the exposure wavelength. Even if an immersion method is employed, the resolution limit is said to be about ¼ of the exposure wavelength, and even if an immersion method of ArF excimer laser (wavelength: 193 nm) is employed, about 45 nm is presumed to be the limit. Under the circumstances, as an exposure technique employing an exposure wavelength shorter than 45 nm, EUV lithography is expected to be prospective, which is an exposure technique employing EUV light having a wavelength further shorter than ArF excimer laser. In this specification, EUV light is meant for light ray having a wavelength within a soft X-ray region or within a vacuum ultraviolet region, specifically for light ray having a wavelength of from about 10 to 20 nm, particularly about 13.5 nm±0.3 nm.
EUV light is likely to be absorbed by all kinds of substances, and the refractive index of substances at such a wavelength is close to 1, whereby it is not possible to use a conventional refractive optical system like photolithography employing visible light or ultraviolet light. Therefore, in EUV lithography, a reflective optical system, i.e. a reflective photomask (EUV mask) and mirror, is employed.
A mask blank is a layered product before pattering, to be used for the production of a photomask. In the case of an EUV mask blank, it has a structure wherein a reflective layer to reflect EUV light and an absorber layer to absorb EUV light, are formed in this order on a substrate made of e.g. glass. As the reflective layer, it is common to use a Mo/Si multilayer reflective film having a molybdenum (Mo) layer as a layer to show a low refractive index to EUV light and a silicon (Si) layer as a layer to show a high refractive index to EUV light alternately stacked to have the light reflectivity improved when the layer surface is irradiated with EUV light.
For the absorber layer, a material having a high absorption coefficient to EUV light, specifically e.g. a material containing chromium (Cr) or tantalum (Ta) as the main component, is used.
In the production of an EUV mask blank or in the production of an EUV mask from an EUV mask blank, there is a step involving heating of the EUV mask blank. For example, cleaning by a chemical reaction by means of a chemical reagent in order to remove defects, cleaning by a mechanical energy by means of ultrasonic waves, inspection of defects by a laser, baking treatment in a patterning step, or thermal load by etching, corresponds to such a step. Further, the EUV mask will be heated also by exposure by means of EUV light at the time of carrying out EUV lithography.
During such a step involving heating, mixing may proceed at each interface between layers constituting a Mo/Si multilayer reflective film. If such mixing proceeds excessively at each interface between layers constituting a Mo/Si multilayer reflective film, the reflection characteristic during irradiation with EUV light, specifically the peak wavelength of reflected EUV light, will change, such being problematic (see Patent Document 1).
The change in the peak wavelength of reflected light is desired to be at most 0.04 nm, since it is a range wherein a precise wavelength control can be carried out in order to obtain wavelength specifications required for exposure. Here, in this specification, expressions “peak wavelength” and “center wavelength” will be used for description, and the center wavelength may be evaluated as the same index as the peak wavelength.
Therefore, in Patent Document 1, in order to prevent a change in the peak wavelength of reflected light by excessive progress of mixing, the temperature for heat treatment after forming the Mo/Si multilayer reflective film is controlled to be at most 160° C.
However, a step of heating at a higher temperature may sometimes be required for such a reason as e.g. cleaning at a high temperature for the purpose of improving the efficiency for removal of defects, thermal load by dry cleaning in vacuum, use of a high output laser for the purpose of improving the sensitivity in inspection of defects, baking treatment at a high temperature corresponding to the type of a resist to be used, increase in thermal load due to improvement in the etching rate, or use of high output EUV light for the purpose of high speed exposure.
With an EUV mask blank disclosed in Patent Document 2, it is said to be possible to effectively prevent mixing during heat treatment and to suppress the change in the peak wavelength to be at most 0.1 nm, by forming a diffusion preventive layer made of a material containing Mo and carbon (C) between a Mo layer and a Si layer constituting a Mo/Si multilayer reflective film.
However, the EUV mask blank disclosed in Patent Document 2 has a problem such that the diffusion preventive layer is formed by reactive sputtering by means of hydrocarbon gas between a Mo layer and a Si layer constituting a Mo/Si multilayer reflective film, whereby control of the composition or the film thickness tends to be complex, and the wavelength control with high precision or maintenance of the periodicity of the film thickness in the multilayer film tends to be difficult.